A technology of estimating a gaze position of a user based on an image of the user's face that is photographed by a single camera has been proposed. Patent Literature 1, for instance, discloses that the center point of a user's iris is detected from an image photographed by a single camera, the projection position of the center of the user's eyeball is further estimated from the image, and the direction of the user's sight line is estimated based on the iris center position and the projection position of the eyeball center.